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Recently we see some changes in the pipeline start success rate.
However, we can only get this statistics indirectly through some other
UMAs. This CL adds a UMA directly for this.

Note that if the media pipeline is destroyed during the starting
process, this UMA will NOT be reported. This should be relatively rare.
Also, in this case, the result won't affect user experience anyways.

TEST=Manually tested to make sure the metric is reported.

Review-Url: https://codereview.chromium.org/2383493004
Cr-Commit-Position: refs/heads/master@{#422623}